Hydraulic Retention Time (HRT)
Calculate the hydraulic retention time (HRT) of a reactor or tank, HRT = volume ÷ flow, dividing the working volume (m³) by the influent flow (m³/h). The result, in hours, is the average time the liquid stays in the unit and is decisive in designing clarifiers, anaerobic reactors, lagoons and aeration tanks: short times prevent reactions or settling from completing, while long times raise cost and footprint. Enter the working volume and the inlet flow.
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Hydraulic retention time (HRT)
The hydraulic retention time (HRT, also called detention time) is the average length of time a portion of liquid stays inside a reactor or tank, and it follows straight from the definition of flow rate: HRT = volume ÷ flow rate. If a 200 m³ tank receives 50 m³ per hour, on average every drop spends 4 hours in there. It is a central parameter in any treatment unit, since the processes at work — particle settling, anaerobic digestion, biological oxidation, contact with a disinfectant — all need time to happen. Too short an HRT keeps the reaction or the settling from completing (efficiency drops); too long an HRT calls for huge tanks, driving up construction cost and taking up land. That is why each process has its own typical range: clarifiers work in hours, anaerobic reactors and lagoons in hours to days. One caveat: HRT is the ideal theoretical time — hydraulic short-circuiting and dead zones make the actual residence time shorter than the calculated one. Enter the working volume and the inflow rate.
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Spillway Discharge (Creager/Ogee)
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Velocity Gradient (Mixing)
Calculate the mean velocity gradient (G) in rapid-mix and flocculation chambers, G = √(P ÷ (μ × V)), from the dissipated power (W), the water dynamic viscosity (Pa·s) and the chamber volume (m³). The result, in s⁻¹, measures mixing intensity: rapid mixing needs high G (700–1000 s⁻¹) to disperse the coagulant, while flocculation uses low G (20–70 s⁻¹) to promote floc collision and growth without breaking them. Enter power, viscosity and volume.
Fire Water Reserve (RTI)
Calculate the fire water reserve volume, V = flow × time ÷ 1000, multiplying the system's required flow (L/min) by the required autonomy time (min) and converting to cubic metres. The result, in m³, is the water volume the tank must keep reserved exclusively for firefighting — sized to feed hydrants and/or sprinklers for the minimum time set by codes (typically 30 to 60 min, depending on risk). It is a central calculation in building firefighting installation design. Enter the flow and the autonomy time.
Silo Emptying Time
Calculate the time to empty a silo by gravity discharge, t = M ÷ W, from the stored product mass M (kg) and the mass discharge rate W (kg/s). Since the discharge rate of a granular material through an orifice is practically CONSTANT (independent of the product height above, by the Janssen effect and per the Beverloo equation), the emptying time is simply total mass divided by rate — a direct relation, unlike a liquid's emptying, which slows as the level falls. This time is an important operational parameter in silo, hopper and storage-unit design and operation: it sets the dispatch capacity (how fast a truck, rail car or ship is loaded), sizes the downstream conveying systems (belts, bucket elevators, screws) that must match the discharge rate, and frames shift logistics and vehicle queues at grain terminals. The discharge rate W can be estimated by the Beverloo equation from the outlet diameter, closing the calculation: larger outlets discharge faster (W ∝ D₀^2.5), reducing emptying time. Enter the stored mass and the discharge rate.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.